This invention relates to a lithium ion energy storage device and a method for ascertaining a depth of discharge and/or a state of charge of a lithium ion energy storage device in accordance with the invention.
Lithium ion energy storage devices have a high cell voltage and a good ratio between stored energy and weight and are therefore suitable for mobile applications, in particular for electrically operated vehicles. Lithium ion energy storage devices are frequently produced in a wound construction, wherein a planar material is wound up, said planar material comprising two planar electrodes and a separator that separates the electrodes electrically from one another. The electrodes typically comprise a metal collector that is coated preferably on both sides with an electrode material. Typically, aluminum foil is used as the metal collector in the cathode electrode and copper foil is used as the metal collector in the anode electrode. Usually, the separator is saturated between the electrodes with an ion-transporting means that renders it possible for the ions to pass through the separator.
It is known to measure the current that is output by a lithium ion energy storage device using a plurality of different sensors, by way of example sensors having a shunt resistor or having Hall sensors. In addition to measuring means for the direct current flow, a plurality of other sensors and/or theoretical models do exist that can be used to ascertain the characteristics of lithium ion energy storage devices. For this purpose, sensors are typically used to ascertain a number of specific characteristics of the energy storage device and the theoretical models are applied to the ascertained values in order to infer the electrical characteristics and/or the state of the energy storage device.
EP 2442400 A1 discloses an electrochemical cell on the basis of lithium technology having an inner-lying reference electrode. This is embedded in the separator so that a reference cell is formed between the reference electrode and each of the conventional electrodes of the cell. By virtue of determining characteristics of the two reference cells that represent half cells of the energy storage device, information can be obtained regarding each of these half cells and consequently also regarding the entire energy storage device. In so doing, either the current of the entire cell is measured or a state of the energy storage device is inferred by way of other parameters and theoretical models. The additional electrode that is introduced into the separator prevents the flow of ions at this site and is encumbered with the risk of a short circuit of the electrodes.